The commercial value of a flow meter is dependent upon its ability to provide strong, clear signals which, for ease of use, are linearly related to fluid parameter signals, to be applicable to a variety of fluids each possessing unique viscosities and be usable over a large range of flow rates. There is, however, a tendency for such devices to produce signals which are not consistently measurable, thus causing signaling devices of lesser accuracy than desired. Many flow meters in a similar classification rely on the principles of Bernoulli's Equation, thus (usually) relating fluid velocities to fluid pressures. A drawback to existing differential pressure measuring flow meter systems is that the entire flow stream is typically necked-down into a reduced flow area thus causing flow rates for all the fluid to increase and therefore, fluid pressures to drop, thus causing a potentially undesirable head loss on the system.
Bernoulli's Equation: ΔP=ρv2/2 Where P is the pressure of the fluid, p is fluid density, and v is the fluid velocity. This allows one to use the effects of an aerodynamic wing in which the speed of the air moving over the top face of a wing causes a lower pressure, creating lift. Conversely, slowing the speed of air moving under the lower face of a wing causes a higher pressure, creating elevated pressure, thus lift. This also allows one to create a higher ΔP from the flow element but have a lower ΔP through the entire flow measuring system.
Poiseuille's Equation: Q=(πr2/8ηL)ΔP Where Q is the flow rate, R is the radius of restriction, n is the fluid viscosity, L is the length of the restriction, and P is the pressure of the fluid. This allows one to minimize the effects of drag and viscosity by elongating the flow passage to reduce turbulence.